Experimental Design and Soil Sampling

The soil samples were collected from Sidi Village, which is situated in a typical karst depression with approximately 0.5 km² of arable land. As we have previously reported, this land was contaminated by the leakage of deposited Pb-Zn tailings in the 1970s due to the collapse of a poorly constructed dam for the collection of Pb-Zn tailings during storm floods; subsequently, the contaminated farmland was directly plowed for farming activities (Jin et al., 2015; Li et al., 2015). After almost 40 years of agricultural utilization, soil conditions in this region present stability. We selected typical contaminated land that is currently in use; the land can be divided into three types of fields: evergreen field with C3 citrus plants, therophyte field with C3 rice plants and therophyte field with C4 corn plants. In addition, due to the limited land with karst characteristics in this region, the other un-plowed fields with C3 plants, which were not directly contaminated with Pb-Zn tailings, were selected as the control group (CK).

According to our experimental design, 13 soil samples from paddy fields, 13 soil samples from corn fields, 14 soil samples from citrus fields and 4 CK soil samples were collected in June 2015. The sampling sites are marked in Figure ​Figure11. Each soil sample (500 g) from three soil cores that were 5 cm in diameter and 20 cm in depth was thoroughly mixed. The soil samples were sieved through a 0.2 mm mesh to remove all rocks and roots. The homogenized samples were stored at 4°C for subsequent analysis.

The sampling location in Sidi Village. Y, corn field; S, paddy field; G, citrus field; CK, control group.